1. Field of the Invention
The present invention relates to a laminated dielectric filter, and more particularly to a laminated dielectric filter for use as a high-frequency filter in a high-frequency radio apparatus such as a portable telephone and antenna duplexer.
2. Description of the Prior Art
FIGS. 1 and 2 show, in perspective, a conventional laminated dielectric filter devised by the inventors of the present application.
As shown in FIG. 1, the laminated dielectric filter has a dielectric layer 10 which supports thereon a plurality of resonant elements 14, 16, 18 spaced apart by predetermined intervals from each other, which constitute quarter-wavelength stripline resonators respectively, the resonant elements 14, 16, 18 having ends connected to a ground electrode 12, and a plurality of electrodes 20, 22, 24 having ends connected to the ground electrode 12 and opposite ends spaced apart by predetermined distances from the open ends of the resonant elements 14, 16, 18, respectively, in confronting relationship thereto, resulting in inductive coupling between the resonant elements 14, 16, 18. The laminated dielectric filter also includes another dielectric layer 26 placed on the dielectric layer 10 and supporting thereon an input electrode 28 which is positioned in overlapping relationship to the resonant element 14 on an input terminal side across the dielectric layer 26 and an output electrode 30 which is positioned in overlapping relationship to the resonant element 18 on an output terminal side across the dielectric layer 26. The laminated dielectric filter further includes still another dielectric layer 32 placed on the dielectric layer 26. The dielectric layers 10, 26, 32 are integrally combined into a laminated assembly 40, as shown in FIG. 2.
In FIG. 2, the ground electrode 12 is disposed on upper and lower surfaces of the laminated assembly 40 and side surfaces thereof except input and output terminal areas 42, 44. The input terminal area 42, which is positioned on one side surface of the laminated assembly 40, has an input terminal 46 that is insulated from the ground electrode 12 and connected to the input electrode 28. The output terminal area 44, which is positioned on an opposite side surface of the laminated assembly 40, has an output terminal 48 that is insulated from the ground electrode 12 and connected to the output electrode 30.
FIG. 3 of the accompanying drawings shows an equivalent circuit of the laminated dielectric filter shown in FIGS. 1 and 2. In FIG. 3, the equivalent circuit includes a capacitance 50 between the resonant element 14 and the input electrode 28, a capacitance 52 between the resonant element 18 and the output electrode 30, a capacitance 54 between the resonant element 14 and the electrode 20, a capacitance 56 between the resonant element 16 and the electrode 22, a capacitance 58 between the resonant element 18 and the electrode 24, an inductance 60 indicative of inductive coupling between the resonant elements 14, 16, and an inductance 62 indicative of inductive coupling between the resonant elements 16, 18. The equivalent circuit of such an arrangement serves as a bandpass filter. The equivalent circuit also includes parallel resonant circuits having respective capacitances 64, 66, 68 and respective inductances 70, 72, 74 which are equivalently converted from the respective resonant elements 14, 16, 18.
The bandpass filter has a desired frequency characteristic such as a bandwidth that is obtained by distributed coupling between the resonant elements 14, 16, 18. However, since such coupling is available only between two adjacent the resonant elements of resonant elements 14, 16, 18, it is impossible to provide an attenuation pole for improving attenuation characteristics. While the attenuation characteristics would be improved by increasing the number of resonant elements used, the increased number of resonant elements would also increase the insertion loss of the bandpass filter.
It has been attempted to provide coupling between nonadjacent resonant elements, other than adjacent resonant elements, in order to form an attenuation pole in the attenuation characteristics. For example, it has been proposed to couple nonadjacent resonant elements to form an attenuation pole in the frequency range above or below the passband of the filter, as disclosed in Japanese laid-open patent publication No. 64-78001.
The proposed filter arrangement is, however, disadvantageous in that it requires coupling coils between resonant elements and capacitive elements to couple nonadjacent resonant elements, in addition to the resonant elements themselves, and hence increased manufacturing costs. Furthermore, manufactured filters of such design suffer from variation of frequency at which an attenuation pole appears. In addition, the filter cannot be reduced in size as it includes a large number of parts.